What is knob and tube wiring?

What is knob and tube electrical wiring?

Knob and tube is the early two-wire electrical system still found in many houses built before about 1930, with separate wires held off the wood framing by porcelain parts. The Wisconsin Historical Society and HUD describe it this way: “The first electrical wiring in most historic houses and buildings in Wisconsin was knob and tube wiring.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. “Knob and tube wiring is a two-wire system with no third, grounding wire like modern electrical systems.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. “The oldest wiring system that may still be acceptable, and one still found fairly often in houses built before 1930, is “knob and tube.”” Source: U.S. Department of Housing and Urban Development (HUD), Office of Policy Development and Research, read 2026-10-29.

What does knob and tube wiring look like?

In an attic, basement or open wall it looks like single wires running apart from each other, wrapped around porcelain knobs nailed to the joists and passing through porcelain tubes set in drilled holes: “The wires are threaded through porcelain tubes placed in holes drilled through the joists and studs to insulate the wires from the combustible wood.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. “In places where a wire needs to bend in a new direction or run along a wood surface, the wire is wrapped around a porcelain knob that also prevents the wire from touching the wood.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. Tacoma Power, the city utility in Tacoma, Washington, says: “Ceramic insulators (knobs) were nailed to joists and studs in attics, walls and, sometimes, under the floor to run wires through unobstructed spaces.” Source: Tacoma Power (City of Tacoma, Washington public utility), read 2026-10-29.

How does knob and tube wiring work?

Each circuit uses two separate conductors, one hot and one neutral, kept at least 2-1/2 inches apart where exposed and cooled by the open air around them, according to the Wisconsin Historical Society and an Underwriters Laboratories history of home wiring: “One wire is “hot,” or active, and the other wire is neutral.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. “The two copper wires run parallel but separate from each other through the floor and ceiling joists as well as stud wall cavities.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. “The individual conductors were run spaced apart at least 2-1/2 inches (if exposed), but as the wires passed through walls and floors, they could be susceptible to dampness and abrasion, which could eventually lead to leakage currents and arcing fires.” Source: Underwriters Laboratories article, published by the City of Idaho Falls, Idaho, read 2026-10-29. “Open wiring systems like knob-and-tube, because they are in open air, can dissipate heat very well, however, in some cases improper or failed splices and joints can “glow” red-hot (as Edison pointed out) and go unnoticed for quite some time.” Source: Underwriters Laboratories article, published by the City of Idaho Falls, Idaho, read 2026-10-29.

When was knob and tube wiring used?

It was the usual way to wire homes from about 1880 to 1950; the phase-out began in the 1930s as sheathed cable such as Romex took over, mostly before World War II: Tacoma Power, the city utility in Tacoma, Washington, says: “The standard method of electrical wiring in homes built between 1880 – 1950 was K&T.” Source: Tacoma Power (City of Tacoma, Washington public utility), read 2026-10-29. “Knob-and-tube wiring systems began being phased out in the 1930’s, probably because of the then growing popularity of non-metallic and armored cable systems for residential buildings.” Source: Underwriters Laboratories article, published by the City of Idaho Falls, Idaho, read 2026-10-29. “Later, non-metallic sheathed cable, known as NMC or Romex, was used in residential and some commercial buildings, largely supplanting knob and tube wiring before World War II.” Source: Indiana Department of Natural Resources, Division of Historic Preservation and Archaeology, read 2026-10-29.

Why is knob and tube wiring dangerous?

The risk comes mostly from age and load: the system was built mainly for lighting, so modern appliances can overload it, and oversized fuses put in to stop blown fuses let the wires carry more current than they were made for: “The main issue with existing knob and tube wiring is the potential for electrical overloading.” Source: Indiana Department of Natural Resources, Division of Historic Preservation and Archaeology, read 2026-10-29. “Originally this wiring was meant to primarily serve lighting needs.” Source: Indiana Department of Natural Resources, Division of Historic Preservation and Archaeology, read 2026-10-29. “Sometimes, as frequently blown fuses became an issue on an overloaded circuit, a higher capacity fuse was used to ‘solve’ the problem by allowing the circuit to carry a heavier load than it was designed to, creating a safety hazard.” Source: Indiana Department of Natural Resources, Division of Historic Preservation and Archaeology, read 2026-10-29. “Knob and tube wiring as seen in this photo should be replaced as it can be a fire hazard.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. In Iowa, the state weatherization standard says: “Unless prohibited by local codes, S-type fuses must be installed in homes having knob and tube wiring, to provide over-current protection.” Source: Iowa Department of Health and Human Services, Bureau of Weatherization, read 2026-10-29.

Does knob and tube wiring have a ground?

No. Knob and tube is a two-wire system with no third grounding wire, which the Wisconsin Historical Society says creates an electrocution hazard that modern grounded cable avoids: “Because knob and tube wiring has no third grounding wire, this system presents an electrocution hazard.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. “All legal electrical wiring today has a copper ground wire running through every cable.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29.

Can knob and tube wiring be covered with insulation?

Only under strict conditions, because the wires were meant to shed heat into open cavities; state rules differ, so the rule where the house is located governs. California's electrical code text and Texas weatherization guidance, and a UL history of 1970s attic fires, show why: In California, the state electrical code text printed on the City of Berkeley's knob-and-tube form reads: “Concealed knob-and-tube shall not be used in: Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating material that envelops the conductors. [CEC §394.12(5)]” Source: City of Berkeley, California, Building and Safety, read 2026-10-29. In Texas, the state weatherization program advises: “Care should be taken during insulation installation to keep it (3 inches minimum distance) from the K&T wiring.” Source: Texas Department of Housing and Community Affairs (TDHCA), Weatherization Assistance Program, read 2026-10-29. “During the energy crisis of the 1970’s, it became popular to blow loose-fill cellulose insulation into the attics and walls of older houses.” Source: Underwriters Laboratories article, published by the City of Idaho Falls, Idaho, read 2026-10-29. “This became a problem for older homes with knob-and-tube wire splices with glowing connections or overheated conductors that could easily ignite this insulation and cause a fire.” Source: Underwriters Laboratories article, published by the City of Idaho Falls, Idaho, read 2026-10-29.

Is knob and tube wiring still legal?

Existing knob and tube is still allowed in many places, but the National Electrical Code has not allowed it in new installations since the mid-1970s, and HUD says it should be replaced during a rehab unless it meets a list of conditions: “Knob-and-tube wiring has not been permitted by the NEC for new installations since the mid-1970’s, however, is still described in the 2005 NEC in Art. 394 for existing installations and by special permission.” Source: Underwriters Laboratories article, published by the City of Idaho Falls, Idaho, read 2026-10-29. “Knob and tube wiring should be replaced during rehabilitation; but if it is properly installed, needs no modification, has adequate capacity, is properly grounded, has no failed insulation, and is otherwise in good condition, it can be an acceptable wiring system and is still legal in many localities.” Source: U.S. Department of Housing and Urban Development (HUD), Office of Policy Development and Research, read 2026-10-29.

How long does knob and tube wiring last?

No published source gives a set lifespan: some knob and tube circuits are still working after more than 100 years, while the insulation on the wires turns dry and brittle with age and heat: “If you look around your historic building, up in the attic or down in the basement, you may find remnants of old wiring and electrical systems, such as ceramic knobs and tubes that have long since been abandoned but not removed or, in some cases, still in service after more than 100 years!” Source: Indiana Department of Natural Resources, Division of Historic Preservation and Archaeology, read 2026-10-29. “The greatest problem with such wiring is its insulation, which turns dry and brittle with age and often falls off on contact, leaving the wire exposed.” Source: U.S. Department of Housing and Urban Development (HUD), Office of Policy Development and Research, read 2026-10-29. “Heat generated by conducting higher electric current accelerates the deterioration.” Source: Indiana Department of Natural Resources, Division of Historic Preservation and Archaeology, read 2026-10-29.

Why is it called knob and tube wiring?

The name comes from its two porcelain parts: knobs that hold the wires along open framing, and tubes that carry the wires through studs and joists: “This system utilizes porcelain insulators (knobs) for running wires through unobstructed spaces, and porcelain tubes for running wires through building components such as studs and joists.” Source: U.S. Department of Housing and Urban Development (HUD), Office of Policy Development and Research, read 2026-10-29. “The very earliest residential wiring systems were an open wiring system often referred to as “knob-and-tube.”” Source: Underwriters Laboratories article, published by the City of Idaho Falls, Idaho, read 2026-10-29.

What is knob and tube wiring made of?

It is made of copper conductors covered in cloth, rubber or paper insulation, sometimes cloth soaked in a flammable asphalt solution, held by porcelain knobs and tubes: “In addition, some original copper knob and tube wires were coated with cotton cloth that was soaked in a flammable asphalt solution.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. “The industry soon switched to rubber coatings, but some original knob and tube wiring could still have this flammable coating.” Source: Wisconsin Historical Society, State Historic Preservation Office, read 2026-10-29. “Early electrical wire used cloth, rubber and paper for insulation; these materials degrade and become brittle with age.” Source: Indiana Department of Natural Resources, Division of Historic Preservation and Archaeology, read 2026-10-29. “These tubes were made of porcelain, with a flange on one end and set on an angle to prevent the tube from sliding through the hole.” Source: Underwriters Laboratories article, published by the City of Idaho Falls, Idaho, read 2026-10-29.

Sources

• Types of Electrical Systems in Historic Buildings — Wisconsin Historical Society, State Historic Preservation Office; read 2026-10-29.
• Residential Rehabilitation Inspection Guide (February 2000), section 5.3 Branch Circuits — U.S. Department of Housing and Urban Development (HUD), Office of Policy Development and Research; published 2000-02; read 2026-10-29.
• Lighting It Up (Preservation at the Crossroads newsletter) — Indiana Department of Natural Resources, Division of Historic Preservation and Archaeology; read 2026-10-29.
• History of GFCI Protection (residential wiring history) — Underwriters Laboratories article, published by the City of Idaho Falls, Idaho; read 2026-10-29.
• Knob & Tube Wiring Inspection Requirements, Form 3504 rev. 2/2018 — Tacoma Power (City of Tacoma, Washington public utility); published 2018-02; read 2026-10-29.
• Form #146 Knob-and-Tube Wiring Safety Report (2025 California Electrical Code), last revised 03/25/26 — City of Berkeley, California, Building and Safety; published 2026-03-25; read 2026-10-29.
• Iowa Weatherization Program Notice 15-01, Weatherization Work Standards (April 9, 2015) — Iowa Department of Health and Human Services, Bureau of Weatherization; published 2015-04-09; read 2026-10-29.
• Best Practice - Knob and Tube Wiring (revised January 3, 2017) — Texas Department of Housing and Community Affairs (TDHCA), Weatherization Assistance Program; published 2017-01-03; read 2026-10-29.